Cathode sputtering etching device with movable guard ring

ABSTRACT

A DEVICE FOR ENGRAVING OBJECTS BY CATHODIC SPUTTERING INCLUDING A CATHODE, WHICH AS THE OBJECT HOLDER, AN ANODE, A VERTICALLY MOVABLE GUARD RING LATERALLY SURROUNDING THE CATHODE, AND AN ANNULAR AUXILIARY ELECTRODE ELECTRICALLY INSULATED FROM THE OTHER ELECTRODES AND SPACED FROM THE CATHODE. A SUITABLE POTENTIAL IS APPLIED TO THE LATTER AUXILIARY ELECTRODE TO CORRECT ION TRAJECTORIES AND THEREBY IMPROVE ENGRAVING PRODUCED BY ION BOMBARDMENT OF THE OBJECT SURFACE.

May 1, 1973 J PQMPE| ETAL CATHODE SPUTTBRING ETCHING DEVICE WITH MOVABLE GUARD RING 2 Sheets-Sheet 1 Filed March 18 1971 nnnnu vac, 1 1 1 Fig.1

May 1, 1973 J PQMPE] ET AL 3,739,873

CATHODE SPUTTERING ETCHING DEVICE WITH MOVABLE GUARD RIM Filed March 18 1971 2 Sheets-Sheet 2 51 WWI/17m 50 50 s 51 Fig.5 g H99 50 Fig.10

n F a w 51 55 51 g 50 55 51 50 United States Patent Oflice 3,730,873 CATHODE SPUTTERING ETCHING DEVICE WITH MOVABLE GUARD RING Jean Pompei and Paul Guiochon, Evreux, France, assignors to US. Philips Corporation, New York, N.Y. Filed Mar. 18, 1971, Ser. No. 125,731 Claims priority, application France, Mar. 18, 1970, 7009665 Int. Cl. C23c 15/00 US. Cl. 204298 9 Claims ABSTRACT OF THE DISCLOSURE A device for engraving objects by cathodic sputtering including a cathode, which serves as the object holder, an anode, a vertically movable guard ring laterally surrounding the cathode, and an annular auxiliary electrode electrically insulated from the other electrodes and spaced from the cathode. A suitable potential is applied to the latter auxiliary electrode to correct ion trajectories and thereby improve engraving produced by ion bombardment of the object surface.

manufacturing electronic microstructures of the integrated-circuit type, for instance. Among other advantages ion engraving has the advantage of reducing the drawback of under-etching involved in an attack of the sidewalls of the cavities during their formation, of smoothing out the sharp angles on the treated surface and of carrying out the operation with higher precision, than the chemical engraving particularly if gaps of very small width have to be made (a few microns).

However it is nevertheless difiicult to obtain perfect regularity of the geometry and of the depth of the engravement regardless of the position occupied by the object on the cathode support, whereas said regularity is needed for accurate reproducibility. In order to obtain said regularity it is known to dispose the object(s) in the central part of the cathode plane, where the majority of ions strikes the target in a direction at right angles thereto and/ or where the density of distribution is substantially uniform. At the cathode periphery there may be found a surface band of greater or smaller width, as the case may be, usually a width of a few centimeters, within which the discharge rate is practically uncontrollable, the ion density varies strongly from one point to the other, the direction of impact of the ions is inclined and the quality of the resultant engravement is poor (under-etching and unevenness in depth).

Many cold-cathode-sputtering devices comprise essentially a cathode supporting the objects to be engraved insulated inside a space formed by a glass bell bearing on a metal slab, which plays the part of an anode. In order to obtain correct results by these devices of simple structure, it is absolutely necessary to group said objects in the central part of the cathode.

Better results over a greater part of the cathode plane can be obtained by a device of the kind disclosed in the specification of the Patent of Addition No. 93,425 to 3,730,873 Patented May 1, 1973 French Pat. No. 1,508,463, which device comprises an anode plate preferably arranged quite near the objects to be engraved and a metal screen laterall surrounding the cathode, while the anode and the screen are brought to a potential equal to that of the base plate of the assembly. Experience shows that with the last-mentioned device it is nevertheless necessary to leave unused a large surface strip along the peripheral part of the cathode. Moreover, the presence of an anode plate very near the cathode hinders visual check of the engraving operation. It is, in addition, desirable or even necessary for said plate to be cooled.

The invention has for its object to provide a device for ion engraving, in which the cathode surface can be utilized almost entirely and a perfect uniformity of engravements can be obtained from one object to the other irrespective of the position occupied by the objects on the cathode.

The invention is based on the discovery that since the irregularities found at the cathode rim are essentially due to stray focusing of the ions as a consequence of the inclined trajectories of the ions relative to the cathode plane, it is possible to obviate said cause of defects by specific corrections of both the geometry and the polarity of the sputtering system near the cathode, which modifies the shape of the equipotential surface.

According to the invention a device for engraving objects by means of ion bombardment produced in a coldcathode space, which device comprises at least one first electrode, termed cathode, supporting said objects and a second electrode, termed anode, the cathode electrode being surrounded laterally by a metallic element termed guard ring, and near said cathode there is placed an auxiliary electrode electrically insulated from said parts of the device.

It is advantageous to position the auxiliary electrode opposite the cathode.

Since the cathode usually has the shape of a disc, the auxiliary electrode should have an annular shape having an internal diameter at least equal to that of the cathode disc. Although the auxiliary electrode of this matching shape is arranged in the active region of the discharge space and is thus capable of directly checking said discharge, it does not hinder visual supervision of the engraving operation. This is an appreciable advantage over the devices described above.

A second advantage of the very presence of the auxiliary electrode regardless of its matching shape resides in the possibility of providing a potential at this electrode, which differs from that of the anode formed, for example, by the slab of the sputtering space and by the guard ring, which is electrically connected to said slab. This enables modification of the polarity of the system and to act in consequence upon the interface between the dark cathode zone and the discharge plasma and hence on the shape of the equipotential surfaces and hence on the direction of impact of the ions on the cathode. By appropriate choice of said potential in accordance with the values of further parameters determining the features of the discharge it can be ensured that said direction of impact is perpendicular to the surface of the cathode over a very large portion thereof. The resultant engraving is then correct, the edges of the cavities made are accurately rectilinear and the depth of engraving is anywhere the same.

It should be noted that auxiliary electrodes have already been used in cold-cathode sputtering devices, particularly in the form of grids, but hitherto the effect expected from the presence of such electrodes has been limited to only the selection of the electrified particles. An example of this selection is given in French patent specification No. 1,586,045, in which a grating positioned in the direct proximity of the anode and brought to a negative potential of a few ten volts relative to the anode arrests or at least brakes the negative-going particles likely to impinge on the substrate support and to damage the same.

With the device according to the invention, particularly designed for ion engraving it will be apparent that on the one hand a check of a phenomenon localized not only on the anode but also in the proximity of the cathode and a problem of homogenisation of cathode sputtering of a target throughout the surface of the target are concerned and on the other hand it is not the intention to select particles, but it is the purpose to correct ion trajectories, while in the third place the target may be of any material and need not be of metal only as in said patent specification No. 1,586,045 and the voltages used for sputtering are direct voltages or high-frequency alternating voltages and need not be direct voltages only.

According to a second feature of the device embodying the invention the guard ring laterally surrounding the cathode is movable in a direction of height relative to the level of the cathode.

The combined use of an auxiliary electrode and a guard ring of variable height provides the advantage of allowing suppression of the focusing effect of the ions at the rim of the cathode and contributes to the obtainment of an engraving profile which is the same to very near the edge of said cathode. Those skilled in the art are quite familiar with this focusing effect which becomes manifest in an increased sputtering intensity and hence in an incomparably deeper engraving distinctly visible over a strip of the cathode having a width of at least 2 cuts, the effect decreasing towards the center of the electrode. With a given discharge rate and in the absence of a voltage at the auxiliary electrode it is found that the importance of the defect develops in accordance with the height of the guard ring with respect to the level of the cathode and hence of the objects to be engraved. In general said importance decreases when the level of the guard ring rises with respect to that of the cathode. On the other hand a very narrow strip of the cathode directly subjacent the peripheral part of the electrode remains completely unattacked by the ions. By a suitable combination of the height of the guard ring and of the voltage applied to the auxiliary electrode it is possible to regulate the ion density in the disturbed zone and to obtain in this manner an overall uniform depth of the engraving.

In practice the ion engraving device embodying the invention is advantageous, since it provides a great freedom in matching different discharge rates and different material to be treated, while it permits utilization of substantially the whole cathode, so that simultaneously more objects or objects of larger size can be treated, and the discharge rate can be stabilized and corrected by a simple adjustment of the voltage applied to the auxiliary electrode, while finally owing to the reduced surface of said electrode the latter is not excessively heated at the high discharge rate and need not necessarily be cooled.

In a particularly important device embodying the invention the cathode. electrode can be disposed at a given level relative to the stationary guard ring. The possibility of varying the height of the cathode further enhances the adaptability of the device according to the invention. The guard ring, which is movable in said device, is machined in the form of a cylindrical crown adapted to slide along a guide wall of cylindrical shape tangential to the internal sidewall of said ring and fastened to the base slab of the assembly. It may be held by three feet at angular distances of 120 from each other, which pass through the slab by means of hermetically closed passages; by means of their screw-threaded portions held in a fixed support with tapped holes outside the space they can be positioned accurately, so that the guard ring held by them can be positioned with the same accuracy.

The objects to be engraved are preferably disposed on a metal plate whose surface facing the discharge space is coated with an aluminum oxide layer (with the exception of the zones for receiving said objects), whereas the opposite surface of said plate bears on the cathode disc.

An aluminum electrode, though being interesting on a first view because it pulverizes with difficulty, involves a disadvantage in ion engraving. Since the secondaryemission coefficient of aluminum is raised by Malter effect owing to the very thin alumina layer always covering the electrode surface, the secondary-electrode flux produced by the ion impact is no longer negligible and reduces accordingly the ion flow at the surface of said electrode. On the one hand this results in a diminution of the energy of the incident ions decelerated by the space charge due to the secondary-emission electrons and on the other hand, for the same reason, the distance of the dark zone increases and exceeds the free mean path of the released aluminum atoms. The likelihood of ionisation of these atoms is very great and the return of the ions to the cathode may result in a contamination of the engraved zones. For these reasons the electrode used in the device embodying the invention is formed, as stated above, by a metal plate (for example stainless steel) covered by a thick layer of alumina (thickness 0.5 to 1 mm).

As compared with the known devices the novel design of the device described above permits employment of a high-frequency alternating voltage for engraving (with a cathode having or not having an alumina coating) or a direct voltage (with a purely metallic cathode). The voltages which may be applied to the auxiliary electrode for regulating the ion distribution and correcting the ion trajectories are low direct or alternating voltages. In practice excellent results have been obtained by voltages of the order of 50 to v.

It is advantageous to include voltage control in the supply lead of the auxiliary electrode. By means of this control it is possible to supervise more effectively the discharge and, by varying the supply voltage of said electrode in one sense or the other, to stabilize the discharge by ensuring that the voltage current has a constant value.

The invention will be described with reference to the accompanying drawing in which:

FIG. 1 shows a schematic vertical sectional view of one embodiment of an ion engraving device in accordance with the invention.

FIGS. 2, 3 and 4 are embodiments of a substratesupport suitable for use in the ion engraving device embodying the invention.

FIG. 5 is a diagram illustrating the influence of the guard ring on the curvature of the equipotential surfaces, the upper part of said guard ring being level with the surface of the substrate-support.

FIG. 6 illustrates the engraving profile resulting from the relative arrangement of the guard ring and the substrate-support shown in FIG. 5.

FIGS. 7, 9 and 11 are diagrams of the same kind as shown in FIG. 5 for a level of the guard ring exceeding that of the substrate-support (FIG. 9) and for the simultaneous use of a guard ring and an auxiliary electrode (FIGS. 7 and 11).

FIGS. 8, 10 and 12 illustrate the engraving profiles resulting from the dispositions shown in FIGS. 7, 9 and 11.

The ion engraving device shown in FIG. 1 is arranged inside a space 10 bounded by a cylindrical glass wall 11 bearing on a metal base 12 and holding itself to a slab 13 on top. The space 10 can communicate on the one hand with a pumping system through a duct 14 opening out in the base 12 and on the other hand with a gas source through the duct 15, fixed to the slab 13. The connections ensuring the hermetic closure, as well as the pumping system and the gas supply are not shown in the figure.

Inside the space 10 are arranged the cathode 16, the

assembly of the guard ring 17 and the guide 18 thereof and an auxiliary electrode 19.

The cathode 16 is formed here by a circular, dished plate, which structure is frequently used in cathode sputtering techniques. A stream of water circulates through the body of the cathode 16 entering through the duct 20, passing around the plate 21 inside said cathode and escaping across the envelope 22, which is concentric with the duct 20. The complete arrangement for holding the cathode is not shown in the drawing; it is represented here only by the insulating ring 23.

The guard ring 17 is shaped in the form of a cylindrical metal crown. This crown is vertically displaceable so that its level may vary with respect to the level of the cathode. For this purpose it may be held on feet formed each essentially by a rod 24 traversing the base 12 through a suitable hermetically closed passage (not shown), which rod is screwthreaded at its lower end 25 and co-operates with a screwthreaded support 26, fixed beneath the base 12. Three of these feet spaced apart by an angular distance of 120 provide an accurate vertical adjustment of the guard ring 17. In order to ensure that the guard ring 17 remains concentric with the cathode 16, a metallic guide piece 18 is provided, along which the guard ring is adapted to slide. This piece 18 is fixed to the base 12. It is associated with a further metal piece 27, arranged opposite the rear side of the cathode 16 at a few millimeters therefrom, which piece prevents the discharge from propagating to the rear side of the cathode.

The auxiliary metal electrode 19 has the shape of a ring. Its mean diameter substantially corresponds with that of the guard ring 17. It is held by a metal arm 28, which passes through the base 12 through an insulating, hermetically closed passage 29.

Opposite the auxiliary electrode 19 the cathode 16 holds the substrate support, which carries the objects to be engraved. This subassembly is advantageously prepared as follows (see FIGS. 2, 3 and 4).

A plate 30 of, for example, stainless steel, is provided with covers 31 of the dimensions of the objects to be machined (FIG. 2). An alumina coating 32 of a thickness of 0.5 to 1 mm. is deposited on the cover 31 and the surface 30a of the plate 30, for example, by a plasma gun (FIG. 3). It is then sufficient to replace the cover 31 by the object. Such an object 33 is shown in FIG. 4 at the place of the central cover.

The alumina coating 32 prevents pulverisation of the metal of the plate 30 and the Malter effect, which may occur in any metal covered by a very fine oxide layer. It is thus not necessary to deposit a metal film on the wall of the space 11, which would render visual controlmeasurements impossible.

A high-frequency alternating-voltage generator represented by the rectangle 34 (see FIG. 1) is connected by its hot terminal 35 in known manner through a capacitor 36 to the cathode 16 of the ion engraving device. The second terminal 37 of said generator is connected to the mass of the system as well as the base 12 and hence also the object 27, the guide 18 and the guard ring 17. For reasons of safety it is preferred to connect the slab 13 also to mass, but this is not necessary for the operation. If conductive objects have to be engraved a direct-voltage generator 34 may be employed instead of a high-frequency alternating voltage supply (it is then not necessary to cover the plate 30 with alumina, while the capacitor 36 is dispensed with) The auxiliary electrode 19 is connected via the arm 28 to a source 38 supplying, for example, a direct voltage. It is advantageous to have a variable direct voltage as is shown in FIG. 1 the variation being obtained with the aid of a potentiometer system, the tapping 39 of which is connected to the arm 28 via a milli-ampere meter 40. A decoupling capacitor 41, connected in parallel with the source 38 and with the supply 40, permits deriving from the mass the high-frequency component of the current circulating through the circuit. The auxiliary electrode thus equipped, which is a virtual biassing electrode, by acting upon the step values of the potential, permits on the one hand of ensuring an impact at right angles to the sunfaces of the objects to be engraved with a given working point and on the other hand of checking during the test the fixity of the working point.

The preliminary operations for producing the discharge carrying out the engraving are performed in the manner usually employed for a cathode-sputtering operation, that is to say, briefly, when the apparatus is arranged in place and the space 10 is closed, first the atmosphere of said space is carefully exhausted, after which argon is introduced for the first time, a second evacuation is carefully carried out and argon is again introduced, the pressure being then maintained by means of a manually or automatically operated control (not shown in FIG. 1) at a value of the order of 10- mm. Hg. The discharge is then initiated and maintained for a variable time in accordance with the depth of the desired engraving. The quality of the resultant engraving partially resides in the absence of contaminants of the discharge gas. For example, a reducing agent (H therein is disadvantageous for engraving binary compounds; an oxidizing agent (0+) is not desirable in the case of metals or semi-conductors apart from any consideration of mask resistance, but on the other hand the presence of nitrogen is required for engraving a nitride likely to dissociate by the impact of argon ions.

In FIG. 5 the shaded horizontal rectangle represents schematically a substrate 50 to be engraved, which completely covers the surface of a cathode surrounded by a guard ring represented by the crown 51, while the upper levels of the substrate and of the guard ring are co-planar. The line 52 indicates the interface between the discharge plasma 53 and the dark cathode zone 54, which is thus at the same potential as said plasma, whereas the line 55 indicates zero potential. The ion trajectories are indicated by the arrows F. The majority of these trajectories F are perpendicularly incident to the substrate, whereas at the periphery of the system the incidence is inclined, the inclination being due to the curvature of the equipotentials. The engraving profile corresponding to such a distribution is shown in FIG. 6, which exhibits a distinct focusing of the ions towards the edge of the substrate, Which becomes manifest in a cavity 56.

The addition of an auxiliary electrode 57 to the structure shown in FIG. 5 substantially at the level of the interface between the plasma and the dark zone, which electrode is at a suitable potential (see FIG. 7) permits acting upon the form of the equipotential 52, particularly at the periphery in a sense such that this equipotential is smoothed. This results in an improved equilibrium of the ion density per surface unit of the substrate and in a correction of the ion paths in the part of the inclined trajectories, 'which results in practice in a more uniform engraving profile (see FIG. 8), free of deep cavities. Along the edge of the substrate over a width limited to about 2 to 5 mms. the depth of engraving is smaller and even quite negligible at the edge.

FIG. 9 shows schematically thegeometry of the engraving device in which the upper level of the guard ring 51 is higher than that of the substrate 50. The arrangement does not materially modify the forms and relative distances of the equipotentials 52 and 55 as compared with those of FIG. 5. However, the level of the equipotential O is further spaced apart from the surface of the substrate 50. The ion paths are also perpendicular to the substrate in the central region and inclined to said substrate at the periphery. The engraving profile shows, how ever, in the absence of an auxiliary electrode, a different shape than that shown in FIG. 6. The guard ring screens and isolates a portion 58 of the bombardment of the order of l to 5 mms. in width at the periphery of the substrate,

while the focusing effect, which also occurs but to a smaller extent, gives rise to the cavities 59 of smaller depth (see FIG. 10). The overall width of the disturbed zone, where engraving is uneven as compared with that at the center of the substrate, is about 10 mms., which is approximately equal to the width of the equivalent zone in the profile of FIG. 6.

The auxiliary electrode at the suitable potential arranged as stated above at a level near the interface between the plasma and the dark zone (see FIG. 11) permits obtaining the desired correction of the ion trajectories, to which corresponds the engraving profile shown schematically in FIG. 12. The depth of engraving is uniform over a very wide surface of the substrate, but it is substantially zero in a peripheral zone 60 of a width of 1 to 5 mms., while the boundary between these two zones is almost sharp in this case.

Consequently, by acting simultaneously upon the height of the guard ring and the value of the potential applied to the auxiliary electrode it can be ensured simply and rapidly, regardless of the discharge conditions, that the quality of the engraving is uniform substantially throughout the cathode surface.

By way of example, when a geometry of the kind shown in FIG. 11 is used, in which the cathode has a diameterof 150 mms., the upper level of the guard ring is at a distance of 5 mms. above the upper level of the substrate and the guard ring is at a distance of 5 mms. from the lateral surface of said cathode, while the distance of the auxiliary electrode above the guard ring is 35 mms. and the engraving is carried out by a high-frequency voltage (power density 1.2 w./cm. frequency 14 mHz.) at an argon pressure of 2x l mm. Hg, a direct potential of +40 v. to earth has to be applied to the auxiliary electrode in order to obtain an engraving profile of the kind shown in FIG. 12, where the non-engraved peripheral zone has a width of mms.

What is claimed is:

1. A device for engraving objects by ion bombardment generated in a cold-cathode discharge, a vessel for containing an ionizable gas and within said vessel a cathode electrode serving as an object holder, an anode electrode, a guard ring laterally surrounding said cathode electrode and movable in a direction of height relative to the level of said cathode, an annular auxiliary electrode electrically insulated from said vessel and electrodes and spaced from said cathode and guard electrodes, and means to apply potentials to said electrodes for producing ion trajectories suitable for engraving.

2. A device as claimed in claim 1 wherein said annular auxiliary electrode is positioned opposite the active surface of said cathode.

3. A device as claimed in claim 1 wherein said auxiliary electrode is connected to a separate source of potential.

4. A device as claimed in claim 3, wherein said potential is unidirectional and has a value which can be progressively varied in a controllable manner.

5. A device as claimed in claim 1 wherein said guard ring is a metal crown surrounding laterally said cathode at a regular distance therefrom and bearing on at least three feet, the height of which is externally adjustable.

6. A device as claimed in claim 5, wherein said crown is a peripheral part of a metal piece along which piece Said crown is adapted to slide.

7. A device as claimed in claim 6 wherein said metal piece comprises a metallic prolongation extending opposite the rear side of said cathode.

8. A device as claimed in claim 1, wherein the object support is metal covered with an alumina layer.

9. A device as claimed in claim 8 wherein said metal support is stainless steel.

References Cited UNITED STATES PATENTS 3,598,710 8/1971 Davidse 204298 3,410,775 12/1968 Vratny 204298 JOHN H. MACK, Primary Examiner S. S. KANTER, Assistant Examiner US. Cl. X.R. 204192 

